It's the first agency-sanctioned treatment for the autoimmune disease that affects the connections between nerves and muscles.

"Patients with LEMS have significant weakness and fatigue that can often cause great difficulties with daily activities," Dr. Billy Dunn, director of the FDA'S Division of Neurology Products, said in an agency news release. "There has been a long-standing need for a treatment for this rare disorder."

LEMS affects about three people per 1 million worldwide, the FDA said. The disorder most commonly occurs among people with cancer, such as small cell lung cancer.

The drug's safety and effectiveness were evaluated in clinical studies involving 64 adults. Those treated with Firdapse were found to have benefited more than those who took a placebo, the agency said.